Device for optionally realizing two mutually complementary functions

ABSTRACT

A switchable compression-expansion device, in which a first and a second impedance are included in the emitter circuits of two emitter followers. The impedances are also included in two isolated signal circuits which are coupled via a current mirror circuit. A selection means maintains one of the two emitter followers in the non-conducting state. The current which is produced in the associated impedance by the conducting emitter follower is coupled to the other impedance by the current mirror arrangement. In the other condition of the selection circuit the reverse occurs so that two complementary transfer functions are realized.

The invention relates to a device for optionally realizing two mutuallycomplementary transfer functions, in particular intended for dynamicnoise reduction systems, comprising a first and a second impedance, afirst and a second follower circuit for optionally applying an inputvoltage across the first and the second impedance respectively, and aselection means for optionally maintaining one of the two followercircuits in a conductive state.

Such a device is specifically intended as a compression-expansioncircuit in dynamic noise reduction systems. If Z₁ is the first impedanceand Z₂ the second impedance, Z₁ /Z₂ for example being the expansionfunction and the inverse function thereof, Z₂ /Z₁ being the compressionfunction. A requirement with which such a device must comply is that thetwo functions should be complementary to the highest possible degreeunder dynamic operating conditions.

From U.S. Pat. Nos. 3,813,559 and 3,829,715, devices are known forrealizing two mutually complementary functions. These known devices arebased on the emitter-follower principle, one signal circuit includingthe series connection of the first impedance, the collector-emitter pathof an emitter follower transistor and the second impedance. Forswitching over from expansion to compression function and vice versa thepatents inter alia propose a device in which in one circuit includes theseries connection of the first impedance, the collector-emitter path ofeither an npn emitter follower transistor or a pnp emitter-followertransistor and the second impedance, the npn or pnp emitter followertransistor being included in the circuit depending on the position ofswitches. If the npn emitter-follower transistor is included in thecircuit, the first impedance is the collector impedance and the secondimpedance the emitter impedance of said transistor, the base electrodeof said transistor being the input terminal and an output voltage beingavailable across the first impedance. If the pnp emitter-followertransistor is included in the circuit, the first impedance is theemitter impedance and the second impedance the collector impedance ofsaid transistor, the base electrode of said transistor being the inputterminal and the output voltage being available across the secondimpedance.

Said known device has the drawback that the two functions have differentdynamic properties because the dynamic properties of pnp-type andnpn-type transistors are different. In addition, it is disadvantageousfor the maximum signal level that the two impedances are included inseries in the circuit. For each impedance substantially half the supplyvoltage is available, so that the dynamic range is halved relative todevices where for each impedance substantially the full supply voltagewould be available.

It is an object of the invention to provide a device which in a simplemanner realizes two mutually highly complementary functions and whichdoes not have said drawbacks, and for this the invention ischaracterized in that the two impedances are included in two individualsignal circuits and the device is provided with a current-mirrorarrangement for selectively coupling the current which is produced inthe associated impedance by the conducting follower circuit to the otherimpedance.

The object of the invention is achieved in that in the two optionalconditions of the device a signal current flows through the sameimpedances and the same types of transistors.

It will be evident that owing to the use of a current mirror arrangementthe device is in particular intended to take the form of an integratedcircuit. Moreover, this has the advantage that the various transistorswhich are eligible for this can be made identical in a particularlysatisfactory manner.

The invention will be described in more detail with reference to thedrawing, in which

FIG. 1 shows a first embodiment of a device according to the invention,

FIG. 2 shows a second embodiment of a device according to the invention,

FIG. 3 shows a third embodiment of a device according to the inventionwith electronic change-over,

FIG. 4 schematically represents an example of an application of a deviceaccording to the invention.

The device of FIG. 1 comprises a first emitter-follower transistor T₁and a second emitter-follower transistor T₂, both of the sameconductivity type, in the present example of the npn type. The bases ofthe transistor T₁ and T₂ are respectively connected to a first inputterminal 1 and a second input terminal 2. The emitters of thetransistors T₁ and T₂ are connected to a first supply line 7 via thefirst impedance Z₁ and the second impedance Z₂ respectively, whichsupply line is for example the earth of the circuit. Moreover, theemitters of the transistors T₁ and T₂ are connected to a first outputterminal 3 and a second output terminal 4 respectively. The collectorsof the transistors T₁ and T₂ are connected to the bases of thetransistors T₃ and T₄ respectively which are of a conductivity typeopposite to the conductivity type of the transistors T₁ and T₂. Theemitters of the transistors T₃ and T₄ are connected to a second supplyline 6, whilst the transistors T₃ and T₄ form a current-mirrorarrangement in that the bases are interconnected. As a result, theemitter currents of the transistors T₃ and T₄ are mutually coupled. Thecollectors of the transistors T₃ and T₄ are connected to the emitters ofthe transistors T₁ and T₂ respectively, so that the impedances Z₁ and Z₂are included in two mutually isolated signal circuits, which are coupledin that the base-emitter junctions of the transistors T₃ and T₄ shunteach other. In order to keep one of the two emitter-follower transistorsin the non-conducting state, the bases of the transistors T₁ and T₂ areconnected to the terminals of a two-way switch 5, whose master contactis connected to the first supply line 7. Said switch 5 is preferably anelectronic switch.

If the situation is as shown in FIG. 1, i.e. the base of transistor T₂is connected to the supply line 7, transistor T₂ is non-conducting andcarries no current. When a signal voltage Vi₁ is applied between thebase of transistor T₁ and supply line 7, said signal voltage will almostfully appear across the impedance Z₁ owing to the emitter-followeraction of transistor T₁ and a signal current equal to Vi₁ /Z₁ will beproduced in said impedance. Transistor T₁ then drives the bases of thecurrent mirror transistors T₃ and T₄, so that said signal current canflow through the collector-emitter path of transistor T₃. If the devicetakes the form of an integrated circuit and the emitter areas of thetransistors T₃ and T₄ are equal, an equal current will flow through thecollector-emitter path of transistor T₄ owing to the current-mirroraction. This current causes a voltage Vu₁ across the impedance Z₂, whichvoltage is available at the output terminal 4 and complies with thefollowing equation:

    vu.sub.1 =(Z.sub.2 /Z.sub.1) Vi.sub.1                      (1)

At output terminal 3 an output voltage Vu₂ is then available for wich:

    Vu.sub.2 =  Vi.sub.1                                       (2)

The inverse function of the transfer function (1) is realized whenswitch 5 is changed over and an input signal voltage Vi₂ between inputterminal 2 and supply line 7. Said signal voltage Vi₂ will almost fullyappear across the impedance Z₂ owing to the emitter follower action oftransistor T₂ and will produce a signal current in said impedance whichis equal to Vi₂ /Z₂. Transistor T₂ then drives the base of thecurrent-mirror transistors T₃ and T₄, so that said signal current canflow through the collector-emitter path of transistor T₄. If the devicetakes the form of an integrated circuit and the emitter areas of thetransistors T₃ and T₄ are equal an equal current will flow through thecollector-emitter path of transistor T₄ owing to the current-mirroraction. This current causes a voltage Vu₂ across the impedance Z₁, whichvoltage is available at the output terminal 3 and which equals:

    Vu.sub.2 = (Z.sub.1 /Z.sub.2)Vi.sub.2                      (3)

At output terminal 4 an output voltage Vu₁ is then available for which:

    Vu.sub.1 = Vi.sub.2                                        (4)

It will be evident that the transfer function (1) and (3) are highlycomplementary in that in both optional conditions of the device signalcurrent flows through the same types of transistors and the sameimpedances.

In principle, a satisfactorily operating device is also obtained if thebase-emitter junctions of the transistors T₃ and T₄ are shunted by adiode or a transistor D₃ which is connected as a diode, as is showndashed in FIG. 1. If the device takes the form of an integrated circuitand if the transistor D₃ which is connected as a diode is identical tothe transistors T₃ and T₄, the currents which flow through theimpedances Z₁ and Z₂ will be distributed uniformly among thecollector-emitter paths of the transistors T₁, T₃ and T₂, T₄respectively. The collector currents of the transistors T₁ and T₂ thenflow almost fully through diode D₃. In an integrated circuit thetransistors T₃ and T₄ and the diode D₃ will generally be formed by alateral pnp transistor with triple collector, two collector electrodesbeing connected to the emitters of the transistors T₁ and T₂ and thethird collector electrode being connected to the base of said lateralpnp transistor.

FIG. 2 shows an alternative embodiment of the device of FIG. 1, adifferent type of current mirror arrangement being employed. The devicecorresponds to the device of FIG. 1 except for the transistors T₃ andT₄, and the various corresponding elements are correspondingly numbered.The collectors of the emitter follower transistors T₁ and T₂ are thenconnected to the bases of the transistors T₅ and T₆ respectively, whosecollectors are connected to the output terminals 4 and 3 respectively,and whose emitters are connected to the supply line 6. The transistorsT₅ and T₆ are of a conductivity type opposite to the conductivity typeof the transistors T₁ and T₂. The circuits in which the impedances Z₁and Z₂ are included are mutually current-mirror coupled in that thebase-emitter junctions of the transistors T₅ and T₆ are shunted by thediodes D₁ and D₂ respectively. In integrated circuits said diodes willgenerally be transistors whose bases are connected to the collectors.

If the situation is as shown in FIG. 2, i.e. the base of transistor T₂connected to the supply line 7, transistor T₂ is in the non-conductingstate and carries no current. As a result, there will be no currentthrough diode D₂ and transistor T₆ will be in the non-conducting state.When a signal voltage Vi₁ is applied between the base of transistor T₁and supply line 7, said signal voltage will appear almost fully acrossthe impedance Z₁ owing to the emitter follower action of transistor T₁and will produce a signal current equal to Vi₁ Z₁ in said impedance.Said signal current flows through diode D₁. If in an integrated circuitdiode D₁ takes the form of a transistor connected as a diode, saidtransistor being identical to transistor T₅, the collector current oftransistor T₅ will be equal to said signal current. Said signal currentthen flows through the impedance Z₂, so that for the output signalvoltage which is available at output terminal 5, the same relationshipis valid for the input signal voltage as in equation (1). At outputterminal 3 a signal voltage is available which complies with equation(2). In a similar way as in the device of FIG. 1, the complementaryequation (3) is realized when switch 5 is set to the other position.

FIG. 3 shows an embodiment of a device according to the invention withan electronic switch 5 and with the impedances Z₁ and Z₂ shown in moredetail. The impedances Z₁ and Z₂ then correspond to FIG. 6 of said U.S.Pat. No. 3,813,559.

In the device of FIG. 3, the emitter-follower transistors T₁ and T₂ areof the pnp conductivity type, whilst the current mirror transistors T₃and T₄ are of the npn conductivity type. The electronic switch is formedby two transistors T₇ and T₈ with a common emitter resistor R₆. The baseof transistor T₈ is connected to a reference voltage source V_(ref) andthe base of transistor T₇ constitutes a control input 8. The collectorsof the transistors T₇ and T₈ are connected to the supply line 6 via theresistors R₂ and R₅ respectively. These resistors R₂ and R₅ arerespectively connected in parallel with the series connection of theresistor R₃ and the base-emitter junction of transistor T₁₀ and theseries connection of resistor R₄ and the base-emitter junction oftransistor T₉. The collectors of the transistors T₉ and T₁₀ areconnected to the base electrodes of the emitter follower transistors T₁and T₂ respectively, which base electrodes are supplied with a biasvoltage by means of the voltage dividers R₉, R₁₀ and R₁₁, R₁₂respectively.

If the voltage at the base of transistor T₇ is greater than thereference voltage V_(ref), transistor T₇ is conductive, whilsttransistor T₈ is conductive if the voltage at the base of transistor T₇is smaller than the reference voltage V_(ref). If transistor T₇ isconducting, transistor T₁₀ is in the conducting state. The collectorcurrent of transistor T₁₀ produces such a high voltage across theresistor R₁₂ that transistor T₂ is turned off. In the similar waytransistor T₁ is in the non-conducting state when transistor T₈ isconducting. Thus, the device is switchable with the aid of a controlvoltage at the base of transistor T₇.

The impedance Z₁ is constituted by a resistor R₂. The impedance Z₂ isconstituted by the series connection of the resistor R₇, thecollector-emitter path of transistor T₁₁ and the resistor R₈. Thetransistor T₁₁ has a collector-base negative feedback via an activenetwork which in FIG. 3 is represented within the dashed block G. Whenit is assumed that transfer function of said network G is g, theimpedance Z₂ will be:

    Z.sub.2 = R.sub.7 + gR.sub.8                               (5)

the network G of FIG. 3 consists of a high-pass filter F₁, whose outputsignal is applied to the base of transistor T₁₁ via an amplifier A₁.Moreover, the amplitude of the output signal of amplifier A₁ ismeasured, in that said output signal is applied to a detector D via asecond amplifier A₁. The output signal of said detector D is a measureof the amplitude of the output signal of amplifier A₁ and is applied toa non-linear filter F₂ with both a frequency and amplitude-dependentcharacter. The output signal of said filter F₂ is applied to a controlinput of the filter F₁ so as to control the cut-off frequency. Thus, thenetwork G provides frequency and amplitude-dependent negative feedbackfor transistor T₁₁, so that the impedance Z₂ has a frequency andamplitude dependent character, which is desirable for dynamic noisereduction systems. A detailed circuit arrangement of the network G isdescribed in each of the cited Patents.

FIG. 4 shows an example of an application of a device according to theinvention, represented by block 12, in a recording and/or playbackapparatus, which illustrates an additional advantage of the use of adevice according to the invention.

The first input terminal 1 of the device according to the invention 12is connected to a recording amplifier 13, to which for example signalsfrom a receiver, microphone or record player are applied. The secondinput terminal 2 is connected to a playback circuit 10 which comprises aplayback head and associated electronic circuitry and which reads thesignals on a record carrier and supplies them to the input terminal 2.The first output terminal 3 is connected to a playback amplifier 11which amplifies the signal at the output terminal 3 and supplies it forexample to a loudspeaker. The second output terminal 4 is connected to arecording circuit 9 which comprises a recording head and associatedelectronic circuitry. Said recording circuit 9 records the signal at theoutput terminal 4 on a suitable record carrier, in particular a magnetictape. Furthermore, the device 12 comprises the supply terminals 6 and 7and a control input 8 for controlling an electronic switch 5 as employedin the device of FIG. 3.

If the device of FIG. 3 is in the recording mode, the first emitterfollower of the device according to the invention will be in theconducting state. A signal which is applied to the input terminal 1 bythe recording amplifier 13 appears at the output terminal 4 modified inaccordance with transfer function (1) and is recorded by the recordingcircuit 9. Simultaneously, the input signal appears unmodified at outputterminal 3, so that during recording of signals the unmodified signalscan be monitored via the playback amplifier 11, for which no additionalcircuits and switches are required.

When the device is in the playback mode, the second emitter follower isin the conducting state. The recording circuit 9 is then renderedinoperative and the playback circuit 10 reproduces a signal which isrecorded on the record carrier. Said signal is applied to the inputterminal 2 of the device 12 and appears at the output terminal 3,modified in accordance with the complementary transfer function (3).This output signal is amplified by the playback amplifier 11.

It is found that the device according to the invention may be includedin a recording and playback apparatus without the use of switches. Modeselection can then be effected with the aid of a control signal. Duringrecording an unmodified monitoring signal is automatically available,for which not a single switch need be reset. The device according to theinvention may take the form of an integrated circuit, only 7 connectionterminals being required, as appears from FIG. 4. Moreover, the deviceaccording to the invention may be incorporated in an integrated circuittogether with for example amplifiers.

What is claimed is:
 1. A device for optionally realizing two mutuallycomplementary transfer functions comprising a first and a secondimpedance, a first and a second emitter follower circuit means foroptionally applying an input voltage applied to said emitter followercircuits across the first and the second impedances respectively, aselection means for optionally maintaining one of the two emitterfollower circuits in the conducting state, the two impedances beingincluded in two separate signal circuits, and a current mirrorarrangement means for selectively coupling the current which is producedin the associated impedance by the conducting emitter follower circuitto the other impedance.
 2. A dynamic noise reduction system comprising adevice as claimed in claim
 1. 3. A device as claimed in claim 1, whereinthe current mirror arrangement comprises a first and a second transistorof mutually the same conductivity type having base-emitter junctionscoupled in parallel, the first and the second emitter follower circuitsrespectively comprise a third and a fourth transistor of a conductivitytype opposite to the conductivity type of the first and the secondtransistors, the collectors of said third and fourth transistors beingcoupled to bases of the first and the second transistors, the emittersof said third and fourth transistors being coupled to the collectors ofthe first and the second transistors respectively, the bases of saidthird and fourth transistors being coupled to a first and second inputterminal respectively, the emitters of said third and fourth transistorsbeing coupled to a first and a second output terminal respectively, thefirst impedance being included in the common circuit of the emitter ofthe third transistor and the collector of the first transistor, thesecond impedance being included in the common circuit of the emitter ofthe fourth transistor and the collector of the second transistor, andthe selection means selectively connects the base of one of the thirdand the fourth transistors in a current-blocking sense to a point ofconstant potential.
 4. A device as claimed in claim 3 further comprisinga recording amplifier having an output coupled to the first inputterminal, a playback amplifier having an input coupled to the firstoutput terminal, a recording circuit having an input coupled to thesecond output terminal, and a playback circuit having an output coupledto the second input terminal.
 5. A device as claimed in claim 1, whereinthe current mirror arrangement comprises a fifth and a sixth transistorof mutually the same conductivity type having base-emitter junctionsshunted by a first and a second semiconductor junction respectively, thefirst and second emitter follower circuits respectively comprising thirdand fourth transistors of a conductivity type opposite to theconductivity type of the fifth and the sixth transistors, the collectorsof said third and fourth transistor being coupled to the bases of thefifth and the sixth transistor respectively, the emitters of said thirdand fourth transistors being coupled to a first and a second outputterminal respectively, the first impedance being included in the commoncircuit of the emitter of third transistor and the collector of thesixth transistor and the second impedance being included in the commoncircuit of the emitter of fourth transistor and the collector of thefifth transistor, and the selection means selectively connects the baseof one of the third and the fourth transistor in a reverse sense to apoint of constant potential.
 6. A device as claimed in claim 5, furthercomprising a recording amplifier having an output coupled to the firstinput terminal, a playback amplifier having an input coupled to thefirst output terminal, a recording circuit having an input coupled tothe second output terminal, and a playback circuit having an outputcoupled to the second input terminal.